Tractor-trailer truck systems vary in length and configuration depending upon the continent upon which the truck operates and depending upon what type of cargo is being hauled. In North America the most common type of truck for hauling purposes, and the type most commonly viewed by motorists is the semi-tractor trailer type of truck, also known as the “18 Wheeler” or simply as a “Semi.” Semi's have been immortalized in many country western songs as written by the likes of Willie Nelson and Ronnie Millsap (e.g. Smoke Mountain Rain), and are part of the highway culture of America.
The Semi is a trailer without a front axle in which a large proportion of the overall weight of the trailer weight is supported by a road tractor, which is a motorized detachable front axle assembly having a “cab” in which a driver positions him/herself to operate the Semi. The tractor is designed to pull a trailer laden with heavy cargo over great distances and deposit “hauled” cargo at a destination point, and to usually then pick-up another trailer having more cargo. A semi-trailer is also normally equipped with “landing gear” which are a set of legs positioned toward the front of the trailer and which can be lowered to support the trailer when uncoupled from the tractor. This allows the trailer to be uncoupled from a tractor and left at is destination for unloading and loading of cargo which can then be hauled by a different tractor.
One issue that faces Semi transportation companies is in what type and size/configuration of trailer should it invest. The most common type of trailer is a “box” or “van” trailer. In North America box trailers have standardized lengths of 28 ft 0 in (8.53 m), 32 ft 0 in (9.75 m), 34 ft 0 in (10.36 m), 36 ft 0 in (10.97 m), 40 ft 0 in (12.19 m), 45 ft 0 in (13.72 m), 48 ft 0 in (14.63 m) and 53 ft 0 in (16.15 m). Other types of trailers also exist, such as a “bus” type; a “curtain sider” semi-trailer; a “flatbed” trailer, which is a type of drop-deck trailer; a “double decker” which has a fixed, hinged or moveable second floor to enable it to carry palletized goods; “dry bulk” trailers that resemble big tankers and are used for hauling dry-goods such as sugar, and flour; a “hopper-bottom” trailer, which is usually used to haul grain; a “livestock” trailer; a “lowboy” trailer which is a type of flatbed trailer in which the load floor is positioned very close to the ground and typically used for heavy equipment such as cranes, bulldozers, etc.; a “reefer” or refrigerator trailer; or a tanker trailer used for hauling liquids such as gasoline, milk, orange juice, and alcohol.
The flatbed trailer is very common site on most interstate highways and consists of a load floor, removable side rails, and sometimes includes a bulkhead on its front to protect the tractor in the event of a load shift. Flatbeds also are commonly configured to receive and haul intermodal containers, sometimes referred to as ISO containers, or shipping containers, A shipping container is a standardized reusable steel box used for the safe, efficient and secure storage and movement of materials and products within a global containerized intermodal freight transport system. They are called “intermodal” because they can be transported on ships, trains, and trucks easily due to standardized size and connection mechanisms. When a freight ship containing a number of shipping containers arrives at a port location, the containers are generally removed by automated crane systems and loaded onto train cars (e.g. a flatcar) or onto a flatbed truck chassis. Each shipping container includes as part of its structure a “twist-lock” receptacle, generally at each corner of the container. The rail cars or truck chassis are configured to support these shipping containers on a flatbed floor or on transverse supporting beams, depending upon the particular configuration of the chassis or railcar, and a twist-lock affixed to the railcar or truck chassis locks each container in place. In case of a truck chassis, twist locks are generally welded onto the chassis support structure near the outer perimeter of the chassis.
However, while shipping containers shipped via truck generally have a standardized size, state regulations vary across North America as to what length of truck chassis must be used for each container being transported, depending upon the weight of each container. A shipping container can be loaded with heavy items, such as large motors or heavy machine parts, thereby sometimes exceeding the weight of the entire semi-trailer. So, some states require that chassis lengths be extended by as much as 15 feet so that the distance between the rear set of wheels and the rear set of wheels of the truck tractor separate the weight over a larger distance. The larger separation reduces the total supportive weight a bridge endures as a truck travels over the bridge by distributing a greater portion of the total truck weight over a greater length of bridge or onto an adjoining roadbed. This can be especially helpful for a relatively short bridge and may allow the structural requirements of a relatively short bridge to be reduced.
Unfortunately, permanently lengthening a truck chassis is less than ideal for hauling goods because shipping containers must generally be positioned toward the middle to the front of a chassis for weight distribution and driving concerns, thereby leaving a wide gap between the end of the shipping container and the end of the chassis. This gap complicates unloading because most trucks back their chassis down into a loading dock that allows for the bed of the chassis, and therefore the shipping container floor, to be positioned flush with the loading dock concrete floor. When flush, the shipping container doors may be opened and workers gain direct access to the goods within the container. Also, some of these goods may be quite heavy and require a loading fork to be used to remove them. Loading forks are relatively heavy and require a firm support such as is generally present at a concrete loading dock.
In response, several attempts have been made to design flatbed and other trailer types to be reconfigurable to adapt to the length of a flatbed to accommodate various sizes of loads being transported. For example, flatbed trailers have been designed to be extendable, such as described in patent publication no. EP0551159 A1 which discloses a trailer with curtain sides and a top with a flatbed floor in two sections. The floor sections are centrally supported on an extendable box-within-a-box chassis, with the forwards floor section sliding over the rearwards floor section as the trailer is retracted. Such a trailer may be loaded by a separate crane or by a forklift truck.
A flatbed trailer was designed by Raven (International Publication WO 00/10834) which allowed the trailer to be extended while connected to and towed by a tractor unit. Raven's trailer included a chassis section with a load bearing first floor surface, supported by a wheeled bogie section. The bogie section provided a second platform for a load bearing second floor surface while being slidably connected to the chassis section.
However, while these and other examples provide for the lengthening of a truck chassis responsive to load size and weight factors, they do not evenly distribute load weight on the wheeled bogie as it extends or retracts. Hence, what is needed is an improved design for an extendable chassis trailer using a roller supported, wheeled bogie.